In a large pot of boiling salted water, cook tortellini according to package instructions; drain.
Meanwhile, in a large skillet, heat oil over medium-low heat. Add mushrooms and garlic; cook, stirring frequently, until mushrooms are tender, about 7 minutes.
Add 1 cup water; season with salt and pepper. Cook until liquid is reduced by half, about 5 minutes. Remove from heat; add tortellini, Parmesan, butter, and parsley. Toss to coat; serve.
If you find fresh tortellini, you can use that instead of frozen; cook in a pot of boiling salted water until the pasta floats to the top, about 3 minutes. The mushroom sauce can also be tossed with plain pasta, such as fettuccine, linguine, or spaghetti. Cook 2 ounces of pasta, and add 2 more tablespoons of Parmesan.